In a time-of-flight (TOF) mass analyzer, as a transient pulse of ions arrives at a detector, it causes the detector to generate an analog output signal whose amplitude is nominally proportional to the number of ions of a particular group. The transit time, measured from the instance when an ion is pushed into a TOF chamber under the influence of an electrostatic push pulse to the time at which the analog ion detector signal is produced, represents the ions' mass-to-charge (m/z) value. A time-of-flight spectrum is produced by summing up the signals from many transient pulses of ions with a data acquisition system capable of handling large amounts of data created within very short time periods.
In the data acquisition system, the analog signal from the ion detector can be digitized with an analog-to-digital converter (ADC) and the digital data is recoded as a function of the transit time to correspond with the m/z values of the detected ions. A waveform capture board with a high sampling rate and on-board memory can be used to perform the analog-to-digital conversion in real time over the range of transit times (mass range) of interest. Typical commercially available waveform digitizers suitable for TOF applications, for example, have a resolution of 8-bits (to give 255 points of analog to digital conversion) and a sampling rate of 1 GHz (providing 1 nanosecond of transit time resolution and the capability of generating 1 GB of data per second).
Generally, an 8-bit, 1-GB/s data digitizer system can provide a response of about four orders of magnitude of resolution. However, in some applications, a wider dynamic range or increased resolution beyond the capability of the current 8-bit digitizers may be desired. For example, when an analysis contains a waveform with a meaningful analog signal having amplitudes less than the lower limit set by the 8-bit voltage comparator, the signal can be overlooked as low level noise. Similarly, an analog signal intensity that is above the 8-bit maximum voltage level may be inaccurately recorded as being equal to the threshold limit and thus affecting quantitation measurements. If the dynamic range of the 8-bit ADC is extended to accept higher analog signals, the resolution will suffer because of the increased coarseness of each conversion step. Potentially, a digitizer with higher resolution capabilities beyond one byte could alleviate this problem but higher resolving ADC's are generally limited to sampling rates of less than 1 GHz operation and/or may be a commercially unfeasible option because of their higher cost and power requirements.
In some cases, one can increase the dynamic range by using two digitizers (analog-to-digital converters or ADC's) simultaneously where each digitizer is set to a different input voltage range. However, using two ADCs simultaneously can generate twice the amount of data since both digitizer produce independently parallel bytes for each digitized point. The volume of data for each analysis can be potentially large and can overwhelm the data processing system. For instance, a push pulse frequency of 80 kHz can be provided by a pulse generator so that 80,000 new spectra can be generated per second. The pulse frequency is chosen according to the length of the flight path so that fast traveling ions from one transient pulse do not overlap with slower ions from the previous transient pulse. While the analog ion detector produces an analog signal as a function of time for each spectrum, the 1 GHz digitizer can divide each analog signal into 1 ns intervals (points) over the total time period of each signal. Typically, the number of intervals over the mass range of interest will determine how well adjacent masses can be distinguished (mass resolution), and the mass range can be defined by the lower and upper transit times calculated according to the flight path of the time-of-flight instrument. In some cases, the difference between the lower and upper transit times can be about 5000 ns and, with a 1 ns digitizing rate, the number of intervals can be in the order of 5000 points. Thus, if two 8-bit digitizers are used simultaneously to collect 5000 interval points for each of the 80,000 spectra per second, the accumulated data for a 1 second spectrum is 6.4×109 bits, or 0.1 GB/s. Since an average acquisition time is about 300 seconds in duration, a single data file created by two 8-bit ADC can be 30 GB or larger. Although data compression can be used to reduce the file size, the raw data can nevertheless be a challenge for the processor's capabilities.